Pressurized gas systems are used in respiratory therapy, in medical procedures and testing, in the breathing apparatuses of divers and firefighters, and in such industrial fields as welding, heating, ventilation, and air conditioning (HVAC), and plumbing. It is important to provide users of these systems with an alarm to indicate that the supply of gas has been or is about to be exhausted, or that the flow of gas has been interrupted. Pressurized gas alarm systems indicate these conditions by means of audible alarms, alarm lights, and the like. Such alarms are especially critical in medical gas uses, where a patient's life may be threatened by the interruption of the flow of oxygen or other gas. In a medical setting, a warning alarm must be perceived not only by the end user of the gas, who may be an incapacitated patient, but also by caregivers, who may be at sites remote from the end user. It is therefore desirable to provide electrically powered alarms, whose warnings can be communicated over distances by wires or by wireless broadcast systems.
A typical pressurized gas system includes a cylinder, tank, canister, or other reservoir, to store a gas at high pressure, and a regulator to allow the gas to flow into a gas line at a constant reduced pressure, and at an appropriate flow rate. The term “pressurized gas system” is defined to include a cylinder or other pressurized reservoir, the regulator, all lines which conduct the gas, and the end use appliance such as a mask, cannula, tent, incubator, or torch. The term “downstream” is defined as the direction of gas flow away from a cylinder or other reservoir. Typically, a gas cylinder includes a main valve and cylinder connector to which a regulator is attached. When the cylinder valve is opened, pressurized gas is admitted into the regulator. Regulators typically include at least two valves. A pressure valve maintains a constant user selected pressure downstream of the cylinder. It maintains that pressure as tank pressure decreases and downstream demand changes, typically by means of a diaphragm-controlled valve. A flow valve, downstream of the pressure valve, regulates the flow rate of gas out of the regulator. It is the flow valve that directly determines the flow rate of gas into a downstream appliance.
There are two types of malfunction that can cause a loss of gas flow at a downstream appliance. The first cause is the exhaustion of gas in the cylinder. Cylinder pressure alarm devices exist in the prior art to provide an alarm indication when a gas cylinder has been exhausted, or when gas pressure in the cylinder has fallen to a predetermined limit. These alarm devices generally include a cylinder pressure sensor that actuates an electronic or mechanical alarm when cylinder pressure reaches a minimum set point. These devices sense gas pressure at a point downstream of the cylinder valve and upstream of the pressure valve of the regulator. It is in this region that tank pressure can be reliably sensed when the cylinder valve is open. Such devices are disclosed in U.S. Pat. No. 6,209,579 to Bowden et al, U.S. Pat. No. 5,040,477 to Schiffmacher, U.S. Pat. No. 6,137,417 to McDermott, and U.S. Patent Application Publication No. US2010/0097232 to Lee et al.
A cylinder pressure sensor, however, is ineffective at detecting the second type of gas flow malfunction: malfunctions that occur in the gas lines downstream of the flow valve of a flow regulator. These downstream malfunctions include the disconnection of a gas line from the flow valve; the disconnection of two joined gas lines; the disconnection of a gas line from an appliance; a leak in a gas line or appliance; and blockages, such as a clog or kink in a gas line.
Cylinder pressure alarms cannot react to these downstream malfunctions. Their pressure sensors are isolated from pressure and flow conditions in the downstream gas lines by at least the pressure valve and flow valve, and in some cases by additional intervening valves. Cylinder pressure alarms are also inapplicable to oxygen concentrators.
Alarm devices which monitor gas flow rate have the potential to detect malfunctions occurring downstream of a regulator, and also to detect depletion of a pressurized gas cylinder or other reservoir. They are also potentially applicable to oxygen concentrators and other devices that employ fans or compressors to generate a gas flow. Disconnections, leaks and blockages, are detectable by gas flow detectors as reductions in gas flow rate by a gas flow sensor located downstream of the malfunction. Disconnections and leaks can also be detected by gas flow sensors upstream of the malfunction, as increases in gas flow rate, which reflect the decreased gas flow resistance caused by a disconnection or leak. The depletion of a gas cylinder or other gas reservoir is also detectable by a gas flow detector situated downstream of a regulator. Even though a regulator buffers the downstream gas lines from changes in cylinder pressure, the near or complete exhaustion of the cylinder will of course produce detectable reduction in gas flow rate downstream. Alarm devices which monitor gas flow are also applicable to oxygen concentrators and other devices that produce gas flow by means of fans or compressors, rather than by means of a pressurized cylinder.
A gas flow alarm device exists in the prior art, but it cannot warn of all malfunctions occurring downstream of a regulator, or of the depletion of a pressurized gas cylinder or other reservoir. U.S. Pat. No. 6,386,196 to Culton discloses a gas flow alarm to detect the detachment of an oxygen line from an oxygen cannula, or between two segments of oxygen line. The alarm consists of a coupler with a proximal end accepting an upstream oxygen line and a distal end connecting to a downstream oxygen line or cannula. The coupler includes an audible alarm, in the form of a whistle at the proximal end. The whistle is normally occluded by the downstream line but is uncovered when the line is disconnected. Upon disconnection, the uncovered whistle, powered by the gas flow from upstream, emits an audible alarm tone. The coupler also includes a visual indication of flow, a small propeller, enclosed in the coupler, which rotates in the gas flow.
The alarm device disclosed by Culton can only sound an alarm in response to a disconnection downstream of the alarm device itself. It cannot sound an alarm if there is a disconnection, leak, or blockage upstream of the alarm device, or if the gas reservoir becomes exhausted. These malfunctions all cut off the gas flow which powers the whistle. The duration of the whistle alert is also limited by the amount of gas available to power the whistle. Furthermore, the whistle can only be perceived by those in the immediate vicinity of the alarm. Should a gas flow malfunction occur upstream of the alarm disclosed by Culton, the only warning is the cessation of rotation of the small enclosed propeller. This cessation is perceivable only by individuals who happen to be scrutinizing the propeller at the time of malfunction. This hardly qualifies as a warning.
There is a need for a gas flow warning alarm that can detect the gas flow malfunctions at any point downstream of a regulator, detects cylinder exhaustion, and produces an alarm indication that is autonomous of gas pressure and perceivable at remote locations and without constant scrutiny of the alarm device. A warning alarm device that detects gas flow malfunctions downstream of a regulator flow valve has one shortcoming. It can provide little advance warning of exhaustion of a gas cylinder or other pressurized gas reservoir. Because a regulator maintains constant flow, exhaustion of the cylinder can be detected only at the point where cylinder pressure has fallen to the point where gas flow ceases. A device that senses cylinder pressure upstream of a regulator pressure valve can be set to provide an alarm at a predetermined pressure, which can be set high enough to provide advance warning of depletion.
When cylinder pressure does drop below a predetermined limit, there may be no one available to perceive an alarm indication or to exchange a depleted cylinder for a fresh cylinder. There is therefore a need for a device that changes the source of a pressurized gas system from a primary reservoir to a reserve reservoir, in response to an alarm indication.